Radiation Dose and Image Quality Assessment of the Bolus Timing Method for CT Angiography

Background: Bolus timing monitoring scans used for computed tomography angiography (CTA) repetitively expose the same region of the body to radiation with a view to determining the appropriate threshold attenuation level of the contrast material (CM). Therefore, for an effective CT scan, it is necessary to minimize repetitive radiation exposures from bolus timing. Objectives: This study aims to effectively identify and reduce the radiation dose needed for repetitive monitoring scans utilized in CT involving the bolus timing method. Patients and Methods: To investigate the currently employed protocols and the number of monitoring scans in bolus timing CT studies, 983 patients (581 males and 402 females; mean age 62.7) were retrospectively analyzed from October 1, 2013 to March 31, 2014. To identify the appropriate kVp and mA change in the hounsfield unit (HU) and measure the contrast-to-noise ratio (CNR) and the signal-to-noise ratio (SNR) in the bolus timing method, a 2 mL syringe containing CM diluted with saline at a ratio of 1 to 300 was inserted into cavities of a CT dose index (CTDI) head phantom and scanned with a 64-section multi-detector row CT system. The dose variation was analyzed using results from the ionization chamber and CTDI. Results: This retrospective analysis of bolus-timing CT studies found that patients underwent an average of 4.5 to 5.4 monitoring scans. Furthermore, while the HU value of the mixed CM syringe in the central CTDI head phantom cavity was inversely proportional to mA, the decrease in the HU value became weaker at a value of 50 mA with 100 kVp and at 30 mA with 120 kVp. Here, the CNR (Pearson correlation r = 0.983, Kendall’s t = 0.978, P < 0.000) became slightly higher with an increase in mA at 100 kVp compared to 120 kVp, whereas the SNR (Pearson correlation r = 0.997, Kendall’s t = 1.0, P < 0.000) became similar to 100 kVp. Conclusion: It is recommended that minimum level of radiation (100 kVp, 50 mA; 120 kVp, 30 mA) at which the HU value at the trigger threshold is not significantly different from that of the actual helical scans should be used to minimize the radiation dose caused by repetitive monitoring scans in the bolus timing method.